1. Field of the Invention
The invention involves methods for production of fine-grain metal carbide powders and the resulting fine-grain powders and more particularly to tungsten carbide (WC) powder.
2. Background Information
Since the early days of industrial cemented carbide production, tungsten carbide (WC) powder has remained an important starting material. For decades tungsten oxide powder or powder of a tungsten oxide-containing compound has been filled into boats for its production, reduced under a reducing atmosphere for more than 10 hours to tungsten powder, the tungsten powder then mixed with carbon powder and again filled into a boat and made to react to tungsten carbide during a process lasting several hours by carbon diffusion into the tungsten grains. Tungsten carbide powder with particle size lower limits of a few xcexcm diameter were produced according to the technically available particle sizes for the tungsten (W) metal obtained by reduction of the oxide.
With further development of improved cemented carbide grades over the past two decades and intensified demand for so-called submicron tungsten powder grades, gas carburizing methods have gained significance in addition to carburizing by carbon diffusion. As the name suggests, carburizing of tungsten in the gas carburizing method occurs by means of carbon transported via the gas phase to the tungsten surface. Methods are also known in which tungsten carburizing occurs both via solid carbon added to the tungsten powder and via carbon-containing gases. A significant reduction in process time and simplification of process control is expected with the application of the gas phase reaction, especially because undesired depositing of free carbon appears to be thermodynamically manageable via control and metering of the gas phase. It should also be expected that in the gas carburizing method, depending on the process temperature, particle enlargement in the already formed WC can be avoided or kept within tolerable limits. Particle enlargement in the already formed WC is unavoidable in the diffusion process as a result of longer process times.
Gas carburizing processes occur in a fluidized bed, ordinarily in rotary tube kilns or fluidized bed chambers. The corresponding installations operate continuously with respect to the product being carburized or in a sequence with stipulated filling amounts.
Coarse-grained dense tungsten carbide powder cannot be technically reduced or ground to powders with a particle size  less than 1 xcexcm. These are so-called submicron powders. It would therefore seem appropriate to arrive at submicron tungsten carbide powder using another technique. One such technique is by using submicron initial powders as the product being carburized via the gas carburizing process.
However, implementing a gas carburizing process for submicron WC powders is not easy. In fact, when the above ideas were implemented, a number of difficulties appeared that find expression in a large number of different proposals for process control.
Among the process developments for production of submicron or nanocrystalline tungsten carbide powder or WCxe2x80x94Co powder by gas carburizing, mostly two methods have been proposed, and will be further described below as representative with reference to U.S. Pat. No. 5,372,797 (the ""797 patent) and European Pat. No. EP B 0 598 040 (the ""040 patent).
A process for production of tungsten carbide with a particle size of 50 to 200 nm is described in the ""797 patent. A solid tungsten-containing starting material is allowed to react in a flowing gas atmosphere containing hydrogen and molecular methane xe2x80x9cuntil nearly all the tungsten starting material is converted to WC.xe2x80x9d The reaction occurs in two process sections, a first at 520xc2x0 to about 550xc2x0 C. and a second with a heating rate of 3xc2x0 to 10xc2x0 C. per min at about 800xc2x0 to about 900xc2x0 C. The temperature rise and partial pressure of the forming water is controlled so that xe2x80x9cthe mentioned powder particle size is produced.xe2x80x9d According to the main claim, at least 50% of the tungsten carbide with a particle size of 50 to 200 nm is to be formed during at least 15 min of reaction time in the second process section. The tungsten carbide so produced has a BET surface from 1 to 10 m2/g, whereas the tungsten-containing starting material should possess a BET surface from 0.01 to 0.09 m2/g, but at the same time commercial xe2x80x9cGTE TO3xe2x80x9d tungsten carbide powder with an average particle size of about 10 to 30 xcexcm is described. According to Example 1 of the ""797 patent, the reaction gas consists of 3% molecular methane and 97molecular hydrogen. The first partial reaction occurs after heating at a heating rate of 20xc2x0 C. per min to 535xc2x0 C. process temperature, the second reaction during a process temperature of 850xc2x0 C. maintained for 90 min, which is controlled with a heating rate of 5xc2x0 C.
According to individual preferred practical examples the process occurs in a horizontal kiln through which the product being carburized passes.
The entire process is controlled by precise monitoring of the weight change of the product being carburized in its partial processes. Following the reduction step, the W powder is essentially completely converted to W2C (see Example 1 of the 797 patent) before WC is formed in another partial process.
According to Example 2 of the ""797 patent the tungsten carbide end product has a total carbon content of 6.13 wt % and the BET surface of the powder was determined at 4.0 m2/g. A shortcoming in the mentioned process is that corresponding experiments were only conducted in a thermobalance unit or in an exceptional case in a mini-laboratory furnace with reaction amounts of up to 20 g. For lack of sufficient control, the critical process parameters of the ""797 patent cannot be scaled up to commercial production. The very complex process control prescribed there, especially the exact temperature control and correlation with H2O partial pressure in the region of the product being carburized, cannot be economically transferred to equipment for industrial tungsten carbide production. For this reason, the method disclosed in the ""797 patent is not technically feasible in ordinary charges of the product being carburized in ordinary industrial units.
The WC powders produced according to the ""797 patent always contain undesired W2C powder fractions detectable in XRD recordings and generally undesired free carbon.
The ""040 patent describes a carbothermic reaction process for production of nanophase metal/metal carbide particles, for example, WCxe2x80x94Co powder. The overall process is divided into three sections, a porous precursor particle being produced after step 1, which acts as substrate for penetration of carbon. In a second step carbon from a gas serving as the carbon source is allowed to penetrate the porous precursor particles at a carbon activity acxe2x89xa71.0 and in a concluding step carbon and the gas serving as the carbon source are simultaneously made to react with the precursor powder particles so that at least one carbide phase is formed and the remaining unconverted carbon is eliminated by gasification with a second gas that can combine with carbon.
The porous precursor particles are produced according to the description in a demanding wet reaction from known tungsten oxide precursors, like H2WO4 or ammonium paratungstate by chemical dissolution and spray drying. Co(en)3WO4 or also AMT-Coxe2x80x94Cl2 are such precursor materials. (en stands for ethylenediamine and AMT is an abbreviation for the costly ammonium metatungstate.)
The carburizing reaction preferably takes place in a fluidized bed. 700 to 850xc2x0 C. is stated as process temperature. CO gas is used as process gas for the WC reaction, which automatically means a carbon activity ac greater than 1. To eliminate formed, unreacted free carbon after the actual WC carburizing process from the reaction zone, the process atmosphere is changed to a mixture of CO and CO2 or CO/H2, as well as CH4H2 with a carbon activity of about 0.5, which permits the conversion C+CO2 to 2CO. It is proposed as an alternative in the ""040 patent that free carbon be added during preparation of the precursor material. According to evaluation by microstructural analysis, WC powder with a particle size between 30 and 100 nm is obtained.
In the process of the ""040 patent, in the case of the WCxe2x80x94Co reaction the known effect that cobalt serves as catalyst for the process step is utilized.
A drawback in the ""040 patent process is the demanding pretreatment of ordinary tungsten oxide-containing preliminary materials via salt solutions to a spray-dried precursor material containing porous tungsten and carbon. A metal/metal carbide mixture, for example, WCxe2x80x94Co, is exclusively produced, which is sometimes desired, but in other cases undesired. Because of the stipulated process control of carbon activities acxe2x89xa61, and regularly much greater than 1, the desired conversion to WC and not to the undesired W2C only occurs at very high total carbon contents of about 20% or more. This again makes the time-consuming final treatment mentioned in the description with CO, CO2 or CO/H2 gas mixtures of low carbon activity (ac less than 1) unavoidable in order to reactively bond and eliminate via the gas atmosphere free carbon unavoidably formed beforehand. In the first process section of WC formation, however, an ac greater than 1 is unavoidable for economic reasons, since at av less than 1 and because of a process temperature of about 800xc2x0 C. (not to be surpassed for reasons of particle size stabilization), unacceptable process times of up to 20 h result.
In principle, a reaction temperature between 700xc2x0 and 850xc2x0 C. means long process times, even with restriction to metal/metal carbide mixture formation utilizing the aforementioned catalytic effect of the metal. The significant shortcomings of the ""040 patent process are therefore the demanding preliminary material preparation and the overall long process time.
For executability of the above process, large-volume granulates must be formed from the fluidizable product being carburized, which in turn can only be ground with difficulty after WC conversion and can technically be deagglomerated at best unsatisfactorily to nanophase particles.
Accordingly, further processing of the WCxe2x80x94Co powder so obtained to useful submicron WC sintered carbide is only possible by applying grain growth inhibitors.
The WC particle size in sintered carbide ordinarily attainable lies at 0.6 to 0.8 xcexcm.
Accordingly, processes are needed to provide tungsten carbide particle sizes and qualities and in the same range as available with sintered processes, but using more commercially viable gas carburization techniques.
In one aspect, the invention concerns a gas carburizing method for production of fine-grained WC powder, which has no foreign phases detectable in XRD analysis, with an average primary particle size  greater than 0.2 xcexcm and  less than 5 xcexcm with a degree of carburizing  greater than 96% at a free carbon content  less than 0.1 wt % in installations for fluidized bed reaction, preferably in a fluidized bed apparatus, starting from ungranulated free-flowing tungsten or tungsten oxide-containing powder with an average particle size d50 greater than 10 xcexcm as the product being carburized and from a carbon-containing process gas in the form of a COx/H2 gas mixture with X=1 and/or 2, or a mixture of hydrocarbon gas and H2, optionally with small amounts of water vapor.
The foregoing specific objects and advantages of the invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages that can be realized. Thus, the objects and advantages of this invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art. Accordingly the present invention resides in the novel parts, constructions, arrangements, combinations and improvements herein shown and described.